1. Field of the Invention
The present invention relates to a process for inhibiting a polymerization in a vacuum system, which is used for purifying an easily polymerizable compound such as (meth)acrylic acid and (meth)acrylate under a reduced pressure. The invention also relates to the easily polymerizable compound purification system.
2. Description of the Related Art
In distillation operations of the easily polymerizable compound such as (meth)acrylic acid and (meth)acrylate, polymerization of the easily polymerizable compound must be avoided during distillation. To this end, the pressure inside a distillation column is reduced and the inside temperature is decreased as much as possible, and a polymerization inhibitor such as hydroquinone and phenothiazine, or a gas containing molecular oxygen is used for inhibiting a polymerization.
FIG. 1 is a schematic illustration of a conventional process for the purification of the easily polymerizable compound. This process includes a purifying section which comprises a distillation column 1 and a condenser 2 and a vacuum section which comprises a steam ejector E1, E2 and E3 for reducing a pressure of a purifying section, a barometric condenser 3, surface condensers 4 and 5 and a ejector seal tank 6.
An easily polymerizable compound is usually distilled in the distillation column 1 under a reduced pressure. The pressure inside the distillation column 1 is controlled by the closing and opening of an air regulating valve 8, while the air regulating valve 8 is controlled on the basis of measurements obtained by a pressure gauge 7. Most of the resulting distilled components is condensed through a condenser 2. The exhaust gas from the condenser 2 flows into a vacuum section by the aid of a steam ejector E1 which is reducing the pressure of a purifying section. The gas usually contains the easily polymerizable compound which is not condensed through the condenser 2 and other noncondensing gases. The gas and steam through the steam ejector E1 are condensed in a barometric condenser 3 by contacting the gas with a cooling liquid directly. The resulting condensate is withdrawn into an ejector seal tank 6 and a remaining gas is sucked into the second steam ejector E2. The gas and steam from the second steam ejector E2 are condensed through a surface condenser 4 and the resulting condensate is withdrawn into the ejector seal tank 6 and the remaining gas is sucked into the third steam ejector E3. In the case of a surface condenser, the gas is condensed without direct contact with a cooling water. The gas from a third ejector E3 is condensed through a surface condenser 5 attached at the downstream side of third ejector E3. Generally, both the resulting condensate and a remaining gas are liquid-sealed by the ejector seal tank 6. In some processes, the gas is not liquid-sealed at a final stage.
FIG. 2 is an illustration of another conventional process for the purification of the easily polymerizable compound. This process includes a purifying section which comprises a distillation column 11 and a condenser 12 and a vacuum section comprising a nash pump 13 as a vacuuming device, a pump15 and a tank 14 and a cooler 17. The easily polymerizable compound is distilled in a distribution column 11 and the resulting distilled compound is condensed through a condenser 12. The exhaust gas from the condenser 12 flows into a vacuum section by the aid of the nash pump 13 reducing a pressure of a purifying section. The gas usually contains the easily polymerizable compound which is not condensed through the condenser 12 and a noncondensing gas. The gas is usually sealed by the liquid which is circulated from a tank 14 by the nash pump 13 and the pump 15.
According to the conventional process, the gas exhausted from a condenser connected to a distillation column contains an easily polymerizable compound such as (meth)acrylic acid and (meth)acrylate. The easily polymerizable compound is liable to polymerize, and the resulting polymer clogs a condenser, a liquid ejector, or a nash pump, and the vacuum section is frequently forced to stop.